`get()` is intended as a replacement for `get_deprecated()` and `get_ptr
()`. The former returns the same value for "key not found" and "key
found and contains `null`" which is ambiguous. The latter returns a raw
pointer which is spooky. Returning `Optional<JsonValue const&>` covers
all the previous uses for these.
The `get_foo()` methods are helpers to make user code less verbose. Most
of the time, we only want a specific type of value: if we want a number
and get a string, we respond the same as if the value was not there at
all. These make that easier to express.
This also adjusts the `has_i32()` method and friends to examine the
value instead of just looking at the underlying type.
This patch adds JsonValue, JsonObject and JsonArray. You can use them to
build up a JsonObject and then serialize it to a string via to_string().
This patch only implements encoding, no decoding yet.
